1. Field of the Invention
The present invention relates to a radio receiver for correcting deviation of an RF (Radio Frequency) tuning frequency due to dispersion in variable capacitance elements, thereby shortening search time.
1. Description of the Prior Art
Radio receivers are heretofore known which are equipped with a control circuit for generating digital data to obtain an optimum receiving state by changing the digital data. Among such kinds of radio receivers, FIG. 1 shows a conventional AM radio receiver in which both the frequency of a local oscillation signal and the tuning frequency of an RF tuning circuit are correspondingly controlled to output data of a control circuit.
In FIG. 1, a received RF signal is frequency-selected by an RF tuning circuit (1) and thereafter amplified by an RF amplifying circuit (2). An output signal of the RF amplifying circuit (2) is converted to a 450 KHz IF (Intermediate Frequency) signal in a mixing circuit (3) in accordance with a local oscillation signal generated by a local oscillation circuit (4) and moreover, the IF signal is amplified by an IF amplifying circuit (5) and then AM-detected by a wave detector (6).
Moreover, when the AM radio receiver in FIG. 1 receives radio waves from a desired broadcasting station, frequency divided data for setting a frequency dividing ratio of a programmable divider included in a PLL control circuit (8) is applied to the programmable divider from a control circuit (7). Because the local oscillation circuit (4) and the PLL control circuit (8) form a PLL, the frequency of a local oscillation signal is locked to a frequency corresponding to the frequency divided data. Moreover, the local oscillation signal is also applied to the mixing circuit (3) and an RF signal from the desired broadcasting station is converted to an IF signal.
Furthermore, the output data generated by the control circuit (7) is supplied to a variable capacitance element (1a) through a fine-tuning circuit (9) and thereby the capacitance value of a tuning element comprising the variable capacitance element (1a) and an inductance element (1b) is controlled and the tuning frequency of the tuning element is coarsely tuned. For example, the whole AM receiving band is divided into 13 bands, the control circuit (7) judges in which band the frequency of a desired broadcasting station is included in accordance with the frequency divided data and coarsely tunes the RF tuning circuit (1) to a frequency included in a corresponding band.
Then, the fine-tuning circuit (9) begins fine tuning in accordance with the output data by finely tuning the tuning frequency of the RF tuning circuit (1) every micro interval, that is, by dividing a band into 64 parts. A received electric-field intensity when set to each tuning frequency is detected by an electric-field detecting circuit (10) and the control data in the variable capacitance element (1a) and data showing the electric field intensity corresponding to the control data are temporarily stored in a storage circuit (11). The control circuit (7) detects the maximum-level electric-field intensity out of the data showing electric field intensities stored in the storage circuit (11) and sets the tuning frequency of the RF tuning circuit (1) to a frequency by which the maximum-level electric-field intensity is obtained. Therefore, it is possible to prevent a tracking error due to the dispersion in variable capacitance elements for setting the tuning frequency of the RF tuning circuit (1).
Radio receivers are heretofore known which are provided with a control circuit comprising a microcomputer or the like for generating digital data to obtain an optimum receiving state by changing the digital data. Among these types of radio receivers, FIG. 1 shows a radio receiver for generating frequency-divided digital data for setting the frequency of a local oscillation signal and changing the tuning frequencies of an RF tuning circuit in accordance with a predetermined received electric-field intensity.
In the case of the radio receiver in FIG. 1, an AM reception band is divided into a plurality of bands to coarsely tune the tuning frequency of the RF tuning circuit (1) to the band. However, when the AM receiving band is roughly divided, the range of fine tuning is widened. Therefore, problems occur that the number of data values for the fine-tuning variable capacitance element (1a) increases and time for fine tuning is lengthened. That is, the length of time until tuning is completed after the tuning is begun is a large problem for a listener and a large factor for radio receiver sets. For example, when dividing an AM receiving band into 13 bands and finely tuning them with 6-bit data, a fine tuning time of approx. 300 msec is required. When a received electric-field intensity of a predetermined level or higher is detected at the time of search, a problem occurs that the search time increases because fine tuning is performed each time.
However, when finely dividing an AM receiving band for coarse adjustment in order to decrease the fine tuning time, if there is a desired broadcasting station around the boundary between divided bands, a problem occurs that the tuning frequency is not included in the bands due to the dispersion in values of the variable capacitance element (1a) and receiving sensitivity deteriorates. Particularly when finely dividing an AM receiving band, the number of boundaries between bands increases. Therefore, the above problem frequently occurs. To solve the problem, it is necessary to control the dispersion in tuning frequencies. However, a new problem occurs that selection of varactor diodes is necessary.
As shown in FIG. 2, it is heretofore known that an AM radio receiver is equipped with a broad-band receiving mode and a narrow-band receiving mode. This type of conventional AM radio receiver eliminates tracking errors by receiving RF signals in a broad band at the time of automatic broadcasting-station selection and prevents erroneous stop during automatic broadcasting-station selection in an undesired broadcasting station due to sensitivity deviation.
In FIG. 2, a received RF signal is amplified by an RF amplifying circuit (31) and thereafter tuned by an RF narrow-band tuning (RF tuning) circuit (32) at the time of normal reception. An output signal of the RF tuning circuit (32) is applied to the mixing circuit (3) through a selection circuit (34) under the state of selecting an output signal of the tuning circuit (32). In the mixing circuit (3), an output signal of the selection circuit (34) and a local oscillation signal of the local oscillation circuit (4) are mixed and converted to an IF signal. The IF signal is limited to a predetermined band by an IF filter (36), amplified by an IF amplifying circuit (37), and thereafter AM-detected by an AM detecting circuit (38).
Moreover, a search start signal showing the start of automatic broadcasting-station selection is applied to a control circuit (41) at the time of automatic broadcasting-station selection and thereby, the control circuit (41) applies a control signal to the selection circuit (34) and the local oscillation circuit (4). The selection circuit (34) selects an output signal of the RF broad-band tuning circuit (30) in accordance with the control signal. Because the frequency selection characteristics of the RF broad-band tuning circuit (30) has a broad band, a tracking error is prevented in which a tuning frequency at the RF stage deviates from a received frequency. Under the above state, received frequencies are changed by changing local oscillation frequencies, an electric field intensity is detected by the broadcasting-station detecting circuit (40) in accordance with an output signal of the IF amplifying circuit (37) whenever received frequencies are changed, and presence of a broadcasting station is detected in accordance with whether a received electric-field intensity has a predetermined value. When a broadcasting station is detected, automatic broadcasting-station selection is stopped in accordance with an output signal of the broadcasting-station selection circuit (40), and the selection circuit (34) selects an output signal of the RF tuning circuit (32) and then it is brought under the normal receiving state.
However, though the RF broad-band tuning circuit (30) is selected at the time of automatic broadcasting-station selection, almost all frequency bands of output signals of the RF amplifying circuit (31) are applied to the mixing circuit (3) without being band-limited because the frequency selection characteristics of the RF broad-band tuning circuit (30) have a wide range. For example, if two signals with a strong electric field are included in many input signals of the mixing circuit (3), the signals of which frequencies are the difference between or sum of the two signals with a strong electric field and moreover, a harmonic signal two times larger than each signal are generated due to the square characteristics of a nonlinear element at the input stage of the mixing circuit (3). When the frequencies of the two signals are represented by fa and fb, respectively, disturbance signals of (fa+fb), (fa-fb), 2.times.fa, and 2.times.fb are generated on the nonlinear element and moreover intermodulation disturbance occurs. The disturbance signals are transmitted to the IF filter (36), IF amplifying circuit (37), and AM detecting circuit (38). There is a problem that a broadcasting station is erroneously detected by the broadcasting-station detecting circuit (4) if the disturbance signals have a high level which results in an erroneous stop, where automatic broadcasting-station selection is stopped even though there is no broadcasting station, occurring during automatic broadcasting-station selection.